The present invention relates to means to improve the dielectric performance of an insulative conduit and a liquid dielectric coolant which flows within the insulative conduit in high voltage equipment (that is, equipment utilizing a power level voltage) and, more particularly, to such means where the improved aspects of dielectric performance comprise the reduction of electrostatic charge buildup in the liquid dielectric coolant and the predetermination of the voltage gradient along the insulative conduit having the liquid dielectric coolant therein.
High voltage electrical equipment has begun to utilize cooling means comprising an insulative conduit having a flow of liquid dielectric coolant therein. In such electrical equipment, it is typical for a high voltage or potential difference to be present between two conductors which are spaced along the length of the insulative conduit and which make contact with the liquid dielectric coolant. Thus, the high voltage is impressed across the dielectric coolant between these conductors. The insulative conduit (and also the liquid dielectric coolant flowing therein) are designed to accommodate the high voltage between the foregoing two conductors. A degradation in the performance of either dielectric can lead to puncturing of the insulative conduit with the attendant loss of coolant function of the electrical equipment.
A problem which has been known to occur regarding degradation of the performance of a liquid dielectric coolant which flows within an insulative conduit is as follows. A liquid dielectric coolant usually contains electrically charged ions, some being positively charged and some being negatively charged. Depending upon the properties of the materials of the insulative conduit and of the liquid dielectric coolant, charged ions of one or the other electrical sign become absorbed on the inner wall of the insulative conduit. The oppositely-charged ions are carried "upstream" in the flow of liquid dielectric coolant, and are neutralized by a conductor of the electrical equipment. The separation of the oppositely-charged ions results in an electrostatic voltage, with respect to the gound of the electrical equipment, appearing along the length of the insulative conduit. Such electrostatic voltage can interact with the high voltage preexisting between two conductors which are spaced along the length of the insulative conduit and which are in contact with the liquid dielectric coolant. The resulting or cumulative voltage along the length of the insulative conduit can exceed the dielectric capacity of the insulative conduit, and thereby degrade the dielectric performance of the insulative conduit. The high voltage impressed across the two spaced apart conductors can then very rapidly or catastrophically deteriorate the insulative conduit and result in puncturing therethrough.
One prior art means for reducing the electrostatic charge buildup in a liquid dielectric coolant involves the introduction of a small amount of ionic additive to the liquid delectric coolant. Such a prior art means is described and claimed in the above-referenced, copending patent application.
A system for the delivery of fuel through an insulative conduit in a jet aircraft has experienced the problem of electrostatic charge buildup resulting in a punctured fuel conduit. It is to be noted, however, that the foregoing insulative conduit was not provided with spaced conductors in contact with the fuel and having a voltage impressed therebetween, as is the case with the above-described high voltage electrical equipment. A means employed to avoid the problem of punctured fuel conduits involves the provision of paths of high conductance in a fuel conduit, as, for example, by providing high conductivity fillers in the material of the fuel conduit. Such a means cannot be utilized for high voltage electrical equipment inasmuch as a high conductance path would adversely interfere with the dielectric properties of an insulative conduit having a flow of liquid dielectric fluid therein and which is subject to a high voltage along the length of the insulative conduit.
It would therefore be desirable to provide means to reduce the electrostatic charge buildup in an insulative conduit having a liquid dielectric coolant therein which means does not impair the dielectric properties of the insulative conduit having the flow of liquid dielectric coolant therein.
A problem regarding degradation of the dielectric performance of the insulative conduit is as follows. The insulative conduit typically comprises a glass-epoxy composition having a very high resistivity on the order of 10.sup.16 to 10.sup.17 ohm centimeters, or even higher. It is difficult to precisely control the constituent elements of such a glass-epoxy composition to achieve a resistivity within a desirable, narrow range. The thickness of the insulative conduit wall also has a bearing on the resistivity of the insulative conduit, although the thickness can usually be well controlled. The resistivity along the length of the insulative conduit determines the voltage gradient along the length of the insulative conduit between the two spaced apart conductors. The voltage gradient is the highest along regions of the insulative conduit having the highest resistivity. The electric "stress" imposed on the insulative conduit varies according to the voltage gradient therealong. Accordingly, a region of high voltage gradient along the insulative conduit will cause a high electric stress to be imposed upon such region of the insulative conduit. This region of high electric stress can cause premature and catastrophic deterioration of the conduit in this region, with the attendant loss of cooling function of the electrical equipment.
A prior art means for overcoming the problem of the variations of resistivity along the insulative conduit involves the lengthening of the insulative conduit, whereby the maximum regions of electric stress along the insulative conduit are reduced to a value that can be accommodated by the insulative conduit. It would be desirable, however, to provide a means for predetermining the voltage gradient along the length of the insulative conduit. This would result in the ability to preclude regions of significantly higher than average electric stress along the insulative conduit, whereby the length of an insulative conduit can be shortened from the length utilized with the foregoing prior art means. The shortening of an insulative conduit would save on costs for the following materials: the insulative conduit, the amount of liquid dielectric coolant required, and the size of a container in which to house the insulative conduit.